Effect of the reignition characteristics on long-flame coal by oxidization and water immersion.

After a coal seam is mined, the coal remaining in the goaf is prone to flooding and spontaneous combustion accidents. To explore the reignition (secondary oxidation) characteristics of long-flame coal after oxidation and water immersion, the experimental methods of thermogravimetric analysis and infrared spectroscopy were used to analyze coal samples of oxidation first and then water immersion (FO) and samples of water immersion first and then oxidization (FI) at different pre-oxidation temperatures. The results showed that the content of main oxygen-containing functional groups (hydroxyl, carbonyl, and carboxyl groups) of the FO120 (oxidation 120 °C first and then water immersion) coal sample increased, and the FI 90 (water immersion first and then oxidization 90 °C) coal sample decreased. Pre-oxidation at 120 °C will slow down the decrease in the extent of low-temperature secondary oxidation TG, as the pre-oxidation temperature increases, the total heat release of the FO coal samples first increase and then decrease, and the heat released is high at 120 °C. The FI coal samples transfer active sites during the water immersion process, and the high pre-oxidation temperature leads to the rapid increase of the speed of the primary active site, which leads to the transformation between the secondary active site and the oxygen-containing group, resulting in the cleavage of the oxygen-containing group and increasing the heat production. Water immersion pre-oxidation performed under different conditions has the dual effects of promoting and inhibiting spontaneous coal combustion. This result provides a theoretical basis for preventing spontaneous combustion in coal-mined areas in shallow coal seams after soaking in water.

Real-world particle and NOx emissions from hybrid electric vehicles under cold weather conditions.

Hybrid electric vehicle (HEV) technology is critical to reduce the impact of the internal combustion engines on air pollution and greenhouse gases. HEVs have an advantage in market penetration due to their lower cost and higher driving range compared to battery electric vehicles (BEVs). On the other hand, HEVs use an internal combustion engine and still emit air pollutants. It is hypothesized that HEV performance is impacted by the weather conditions as a result of many factors. It was beyond the scope of this work to systematically evaluate all factors so instead we measured emissions from two vehicles driving city and highway routes in Minneapolis, Minnesota in the winter (-5 °C) and looked for major differences in emissions relative to each vehicle and relative to results that would be obtained from a chassis dynamometer in a controlled laboratory setting at a higher temperature approximately 20 °C). The study then looked to associate differences in emissions with the prevailing conditions to gain new insights. Emissions of interest included the total particle number (TPN), solid particle number (SPN), particulate matter mass (PM), and NOx. One key difference in vehicle engine technology was PFI (port fuel injection) versus GDI (gasoline direct injection). We found the frequency at which the Prius hybrid engine reignited was much higher than the Sonata for city and highway driving, although for both vehicles the catalyst temperature remained high and appeared to be unaffected by the reignitions, despite the cold weather. For most conditions, the Prius emitted more NOx but fewer particles than the Sonata. In some cases, NOx and particle emissions exceeded the most comparable laboratory-based emissions standards.

Superhydrophobic and oleophobic ultra-fine dry chemical agent with higher chemical activity and longer fire-protection.

The re-ignition of pool fires is a common hazard phenomenon in fire extinguishing. Dry chemicals with oleophobicity may solve this problem because powders can float on the oil surface and prevent evaporation of fuel pool. In this research, MAPP (modified ammonium polyphosphate) with superhydrophobicity, oleophobicity, and higher chemical activity is prepared which can quickly quench pool fires and provide longer protection. The activation indexes of MAPP for water, diesel, aviation kerosene and gasoline are 98.5%, 87.4%, 98.7% and 98.4%, respectively. Lower activation energy of MAPP means that it will show higher chemical activity in fire. The fire-extinguishing performance of MAPP is much higher than that of Commercial UDCA (ultra-fine dry chemical agent) during fire experiments. After extinguished by MAPP, the fuel pool is hard to be re-ignited. The significance of this study is to propose a new strategy for preventing the re-ignition of pool fires.

Periodic Partial Extinction in Acoustically Coupled Fuel Droplet Combustion.

This experimental study explored the response of burning liquid fuel droplets to one-dimensional acoustic standing waves created within a closed, atmospheric waveguide. Building upon prior droplet combustion studies quantifying mean and temporal flame response of several alternative fuels to moderate acoustic excitation (Sevilla-Esparza, et al., Combustion and Flame, 161(6):1604-1619, 2014), the present work focused on higher amplitude acoustic forcing observed to create periodic partial extinction and reignition (PPER) of flames enveloping the droplet. Detailed examination of ethanol droplets exposed to a range of acoustic forcing conditions (frequencies and amplitudes in the vicinity of a pressure node) yielded several different combustion regimes: one with sustained oscillatory flames, one with PPER, and then full extinction at very high excitation amplitudes. Phase-locked OH* chemiluminescence imaging and local temporal pressure measurements allowed quantification of the combustion-acoustic coupling through the local Rayleigh index. Similar behavior was observed for JP-8 and liquid synthetic fuel derived via the Fischer-Tropsch process, but with quantitative differences based on different reaction time scales. Estimates of the mean and oscillatory strain rates experienced by the flames during excitation assisted with interpreting specific relationships among acoustic, chemical, and fluid mechanical/straining time scales that can lead to a greater understanding of PPER.

Modeling flame extinction and reignition in large eddy simulations with fast chemistry.

This work seeks to support the validation of large eddy simulation models used to simulate fire suppression. The emphasis in the present study is on the prediction of flame extinction and the prevention of spurious reignition using a fast chemistry, mixing-controlled combustion model applicable to realistic fire scenarios of engineering interest. The configuration provides a buoyant, turbulent methane diffusion flame within a controlled co-flowing oxidizer. The oxidizer allows for the supply of a mixture of air and nitrogen, including conditions for which oxygen-dilution in the oxidizer leads to flame extinction. Measurements to support model validation include local profiles of thermocouple temperature and oxygen mole fraction, global combustion efficiency, and the limiting oxygen index. The present study evaluates the performance of critical-flame-temperature-based extinction and reignition models using the Fire Dynamics Simulator, an open-source fire dynamics solver. Alternate model cases are explored, each offering a unique treatment of extinction and reignition. Comparisons between simulated results and experimental measurements are used to evaluate the capability of these models to accurately describe flame extinction. Of the considered cases, those that include provisions to prevent spurious reignition show excellent agreement with measured data, whereas a baseline case lacking explicit reignition treatment fails to predict extinction.